Annotate const fields as const

[jakobnissen]

Let's see if anything downstream breaks. I don't think it should.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.48%. Comparing base (95d9218) to head (2e18425).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
- Coverage   90.87%   90.48%   -0.39%     
==========================================
  Files          31       31              
  Lines        2400     2397       -3     
==========================================
- Hits         2181     2169      -12     
- Misses        219      228       +9     

Flag	Coverage Δ
unittests	90.48% <100.00%> (-0.39%)	⬇️

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